Positive working photoresist compositions in general use comprise an alkali-soluble resin binder, e.g., a novolak, and a naphthoquinonediazide compound as a photosensitive material.
Such a positive working photoresist is applied on a substrate such as a semiconductor wafer, glass, a ceramic, or a metal by spin coating or roller coating at a thickness of from 0.5 to 2 .mu.m. The coating is dried by heating, and then irradiated with, e.g., ultraviolet ray through an exposure mask to cure the coating in the form of, e.g., a circuit pattern. If desired, the exposed coating is baked. Thereafter, the coating is developed to form a positive image. By etching the substrate using this positive image as a mask, the surface of the substrate can be pattern-wise processed.
The novolak resin thus used as a binder not only is soluble in an aqueous alkali solution without being swelled by the solution, but also forms an image-bearing film which can be used as an etching mask having high resistance particularly to plasma etching. Consequently, novolak resins are especially useful in this application. On the other hand, the naphthoquinonediazide compound, which is used as a photosensitive material, is unique in that it itself functions as a dissolution inhibitor to reduce the alkali solubility of the novolak resin while it decomposes upon irradiation with light to yield an alkali-soluble substance thereby serving to enhance, rather than reduce, the alkali solubility of the novolak resin. Thus, due to this large change in its property upon light irradiation, naphthoquinonediazide compounds are useful especially as a photosensitive material in positive working photoresists.
From the standpoints described above, a large number of positive working photoresists comprising a novolak resin and a photosensitive naphthoquinonediazide compound have so far been developed and put to practical use. In particular, the recent progress of resist materials in improvements for higher resolving power is remarkable, and such improved resist materials have succeeded in giving sufficient results in processing for forming patterns having line widths as small as the order of submicrons.
It has conventionally been thought that use of a resist having a high contrast (.gamma. value) is advantageous for heightening the resolving power and reproducing images with a satisfactory pattern shape, and techniques for obtaining resist compositions suited for such purposes have been developed. These techniques are disclosed in a very large number of publications. In particular, with respect to the novolak resins used as a major component of positive working photoresists, many patent applications have been made concerning monomer composition, molecular weight distribution, synthesis method, etc., and such techniques have succeeded in attaining a certain degree of satisfactory results. Also with respect to the photosensitive material as the other major component, a large number of compounds of different structures regarded as effective in attaining higher contrast have been disclosed. As a result, it has become possible to utilize these techniques to develop an ultrahigh-resolution positive working photoresist capable of resolving a pattern having a size almost equal to the wavelength of a light.
However, the degree of integration in integrated circuits is becoming higher increasingly, and the production of substrates for semiconductor circuits, e.g., VLSI's (very large scale integrated circuit), has come to necessitate a processing for forming an ultrafine pattern having a line width of 0.5 .mu.m or smaller. The photoresists for use in this application are required to attain high resolving power especially stably and have a wide development latitude so as to ensure a constant pattern line width always. The photoresists are also required not to leave a residue on the developed resist pattern so as to prevent the circuit from having processing defects.
On the other hand, it has been found that in the formation of an ultrafine pattern, in particular, one having a line width of 0.5 .mu.m or smaller, there is a phenomenon in which even though a certain degree of resolving power is obtained at a certain coating film thickness, this resolving power deteriorates upon a slight change in coating film thickness (hereinafter, this phenomenon is referred to as "film thickness dependence"). It has surprisingly been also found that when the film thickness changes by only a few hundredths of a micrometer, the resolving power changes considerably, and that all the representative positive working photoresists currently on the market more or less show this tendency. Specifically, when the thickness of an unexposed resist film varies from the intended film thickness in the range of .lambda./4 n (.lambda. is exposure wavelength and n is the refractive index of the resist film at that wavelength), the resulting resolving power fluctuates accordingly.
The presence of this problem of film thickness dependence was pointed out in, e.g., SPIE Proceedings, Vol. 1925, p. 626 (1993), where there is a description to the effect that the film thickness dependence is caused by multiple light reflection in the resist film.
This film thickness dependence has been found to be enhanced in most cases particularly when resist contrast is heightened so as to obtain a high resolving power and a pattern having a rectangular section. In the actual processing of a semiconductor substrate, a pattern is formed from a resist film whose thickness slightly varies from part to part due to the surface roughness of the substrate and unevenness of coating thickness. Therefore, this film thickness dependence has been an obstacle to the pattern formation in which a positive working photoresist is used to conduct fine processing at a resolution close to its resolution limit.
To heighten resolving power, many photosensitive material obtained by the reaction of a polyhydroxy compound having a specific structure with a 1,2-naphthoquinonediazide compound have been proposed so far. These compounds are disclosed in, e.g., JP-A-57-63526, JP-A-60-163043, JP-A-62-10645, JP-A-62-10646, JP-A-62-150245, JP-A-63-220139, JP-A-64-76047, JP-A-l-189644, JP-A-2-285351, JP-A-2-296248, JP-A-2-296249, JP-A-3-48249, JP-A-3-48250, JP-A-3-158856, JP-A-3-228057, JP-A-4-502519 (Tokkohyo), U.S. Pat. Nos. 4,957,846, 4,992,356, 5,151,340, and 5,178,986, and European Patent 530 148. (The terms "JP-A" as used herein means an "unexamined published Japanese patent application".) However, these photosensitive material are insufficient from the standpoint of reducing the film thickness dependence.
On the other hand, it has been known that a resist having high contrast and high resolving power is obtained by utilizing a photosensitive material having a hydroxyl group in the molecule. This technique is described in, e.g., JP-B-37-18015, JP-A-58-150948, JP-A-2-19846, JP-A-2-103543, JP-A-3-228057, JP-A-5-323597, JP-A-6-148878, JP-A-6-167805, JP-A-6-202321, U.S. Pat. Nos. 3,061,430, 3,130,047, 3,130,048, 3,130,049, 3,102,809, 3,184,310, 3,188,210, and 3,180,733, West German Patent 938233, SPIE Proceedings, Vol. 631, p.210, SPIE Proceedings, Vol. 1672, p. 231 (1992), SPIE Proceedings, Vol. 1672, p. 262 (1992), and SPIE Proceedings, Vol. 1925, p. 227 (1993). (The term "JP-B" as used herein means an "examined Japanese patent publication".)
In JP-A-3-228057, the present inventors proposed the following three methods for preparing such a photosensitive material having a hydroxyl group in the molecule.
1. An ordinary polyhydroxy compound is reacted with 1,2-naphthoquinonediazide-5-(and/or -4-)sulfonyl chloride by esterification reaction to obtain a mixture of various esters, and the desired ester (photosensitive material having a hydroxyl group in the molecule) is isolated from the mixture by column chromatography.
2. Selective reactivity with 1,2-naphthoquinonediazidesulfonyl chloride is imparted to hydroxyl groups of a polyhydroxy compound by means of an electronic or steric effect to thereby enable the polyhydroxy compound to undergo a selective esterification reaction.
3. A predetermined proportion of the hydroxyl groups of a polyhydroxy compound are protected, and the remaining hydroxyl groups are reacted with 1,2-naphthoquinonediazidesulfonyl chloride. After the esterification, the protecting groups are removed to recover hydroxyl groups.
Although Method 1 is possible on an experimental scale, Methods 2 and 3 are suitable for industrial production. However, Method 3 has a problem concerning the stability of the 1,2-naphthoquinonediazide group under the conditions used for eliminating the protecting groups. Hence, Method 2 is the most desirable.
Examples of the selectively esterifiable polyhydroxy compound used in Method 2, which has been obtained by imparting selective reactivity with 1,2-naphthoquinonediazidesulfonyl chloride to its hydroxyl groups by means of an electronic or stearic effect, include the following compounds [III] to [V], which are described in JP-A-2-19846, JP-A-2-103543, SPIE Proceedings, Vol. 1672, p. 231 (1992), and SPIE Proceedings, Vol. 1925, p. 227 (1993). ##STR2##
Use of compound [III] or [IV] gives a diester in high yield, while use of compound [V] gives a triester in high yield.
Examples of polyhydroxy compounds which produce a similar effect are described in JP-A-3-228057 and SPIE Proceeding, Vol. 1672, p. 262 (1992), i.e., compounds [VI] and [VII] shown below and the compounds represented by the following formulae [VIII] and [IX]. ##STR3##
In formulae [VIII] and [IX], R.sub.1, R.sub.2, and R.sub.4 each represents a hydrogen atom, a halogen atom, an alkyl group, a cycloalkyl group, an aryl group, or an alkoxy group (provided that the benzene rings to which R.sub.1, R.sub.2, or R.sub.4 is bonded each has at least one hydrogen atom at the ortho position with respect to the hydroxyl group);
R.sub.3 and R.sub.5 each represents a hydrogen atom, an alkyl group, a cycloalkyl group, an aryl group, or an alkoxy group; PA1 R.sub.6 represents an alkyl group, a cycloalkyl group, an aryl group, or an alkoxy group; PA1 R, R', R", R.sub.0, R.sub.0 ', and R.sub.0 " each represents a hydrogen atom or an alkyl group (provided that R and R.sub.0, R' and R.sub.0 ', and R" and R.sub.0 " may be bonded to each other to form a ring); and PA1 x, y, and z is from 1 to 3. PA1 polyhydroxyphenyl alkyl ketones such as 2,3,4-trihydroxyacetophenone, 2,3,4-trihydroxyphenyl pentyl ketone, and 2,3,4-trihydroxyphenyl hexyl ketone; PA1 bis((poly)hydroxyphenyl)alkanes such as bis(2,4-dihydroxyphenyl)methane, bis(2,3,4-trihydroxyphenyl)methane, bis(2,4-dihydroxyphenyl)propane-1, bis(2,3,4-trihydroxyphenyl)propane-1, and nordihydroguaiaretic acid; PA1 polyhydroxybenzoic esters such as propyl 3,4,5-trihydroxybenzoate, phenyl 2,3,4-trihydroxybenzoate, and phenyl 3,4,5-trihydroxybenzoate; PA1 bis(polyhydroxybenzoyl)alkanes or bis(polyhydroxybenzoyl)arenes, such as bis(2,3,4-trihydroxybenzoyl)methane, bis(3-acetyl-4,5,6-trihydroxyphenyl)methane, bis(2,3,4-trihydroxybenzoyl)benzene, and bis(2,4,6-trihydroxybenzoyl)benzene; PA1 alkylene glycol di(polyhydroxybenzoate)s such as ethylene glycol di(3,5-dihydroxybenzoate) and ethylene glycol di(3,4,5-trihydroxybenzoate); PA1 polyhydroxybiphenyls such as 2,3,4-biphenyltriol, 3,4,5-biphenyltriol, 3,5,3',5'-biphenyltetrol, 2,4,2',4'-biphenyltetrol, 2,4,6,3',5'-biphenylpentol, 2,4,6,2',4',6'-biphenylhexol, and 2,3,4,2',3',4'-biphenylhexol; PA1 bis(polyhydroxy) sulfides such as 4,4'-thiobis(1,3-dihydroxy)benzene; PA1 bis(polyhydroxyphenyl) ethers such as 2,2',4,4'-tetrahydroxydiphenyl ether; PA1 bis(polyhydroxyphenyl) sulfoxides such as 2,2',4,4'-tetrahydroxydiphenyl sulfoxide; PA1 bis(polyhydroxyphenyl) sulfones such as 2,2',4,4'-tetrahydroxydiphenyl sulfone; PA1 polyhydroxytriphenylmethanes such as tris(4-hydroxyphenyl)methane, 4,4',4"-trihydroxy-3,5,3',5'-tetramethyltriphenylmethane, 4,4',3",4"-tetrahydroxy-3,5,3',5'-tetramethyltriphenylmethane, 4,4',2",3",4"-pentahydroxy-3,5,3',5'-tetramethyltriphenylmethane, 2,3,4,2',3',4'-hexahydroxy-5,5'-diacetyltriphenylmethane, 2,3,4,2',3',4',3",4"-octahydroxy-5,5'-diacetyltriphenylmethane, and 2,4,6,2',4',6'-hexahydroxy-5,5'-dipropionyltriphenylmethane; PA1 polyhydroxyspirobiindanes such as 3,3,3',3'-tetramethyl-1,1'-spirobiindane-5,6,5',6'-tetrol, 3,3,3',3'-tetramethyl-1,1'-spirobiindane-5,6,7,5',6',7'-hexol, 3,3,3',3'-tetramethyl-1,1'-spirobiindane-4,5,6,4',5',6'-hexol, and 3,3,3',3'-tetramethyl-1,1'-spirobiindane-4,5,6,5',6',7'-hexol; PA1 polyhydroxyphthalides such as 3,3-bis(3,4-dihydroxyphenyl)phthalide, 3,3-bis(2,3,4-trihydroxyphenyl)phthalide, and 3',4',5',6'-tetrahydroxyspiro[phthalide-3,9'-xanthene]; PA1 flavonoid pigments such as morin, quercetin, and rutin; PA1 the polyhydroxy compounds described in JP-A-4-253058, including .alpha.,.alpha.',.alpha."-tris(4-hydroxyphenyl)-l,3,5-triisopropylbenzene, .alpha.,.alpha.',.alpha."-tris(3,5-dimethyl-4-hydroxyphenyl)-1,3,5-triisop ropylbenzene, .alpha.,.alpha.',.alpha."-tris(3,5-diethyl-4-hydroxyphenyl)-1,3,5-triisopr opylbenzene, .alpha.,.alpha.',.alpha."-tris(3,5-di-n-propyl-4-hydroxyphenyl)-1,3,5-trii sopropylbenzene, .alpha.,.alpha.',.alpha."-tris(3,5-diisopropyl-4-hydroxyphenyl)-1,3,5-trii sopropylbenzene, .alpha.,.alpha.',.alpha."-tris(3,5-di-n-butyl-4-hydroxyphenyl)-1,3,5-triis opropylbenzene, .alpha.,.alpha.',.alpha."-tris(3-methyl-4-hydroxyphenyl)-1,3,5-triisopropy lbenzene, .alpha.,.alpha.',.alpha."-tris(3-methoxy-4-hydroxyphenyl)-1,3,5-triisoprop ylbenzene, .alpha.,.alpha.',.alpha."-tris(2,4-dihydroxyphenyl)-1,3,5-triisopropylbenz ene, 1,3,5-tris(3,5-dimethyl-4-hydroxyphenyl)benzene, 1,3,5-tris(5-methyl-2-hydroxyphenyl)benzene, 2,4,6-tris(3,5-dimethyl-4-hydroxyphenylthiomethyl)mesitylene, 1-[.alpha.-methyl-.alpha.-(4'-hydroxyphenyl)ethyl]-4-[.alpha.,.alpha.'-bis (4"-hydroxyphenyl)ethyl]benzene, 1-[.alpha.-methyl-.alpha.-(4'-hydroxyphenyl)ethyl]-3-[.alpha.,.alpha.'-bis (4"-hydroxyphenyl)ethyl]benzene, 1-[.alpha.-methyl-.alpha.-(3',5'-dimethyl-4'-hydroxyphenyl)ethyl]-4-[.alph a.,.alpha.'-bis(3", 5" -dimethyl-4"-hydroxyphenyl)ethyl]benzene, 1-[.alpha.-methyl-.alpha.-(3'-methyl-4'-hydroxyphenyl)ethyl]-4-[.alpha.',. alpha.'-bis(3"-methyl-4"-hydroxyphenyl)ethyl]benzene, 1-[.alpha.-methyl-.alpha.-(3'-methoxy-4'-hydroxyphenyl)ethyl]-4-[.alpha.', .alpha.'-bis(3"-methoxy-4"-hydroxyphenyl)ethyl]benzene, 1-[.alpha.-methyl-.alpha.-(2',4'-dihydroxyphenyl)ethyl]-4-[.alpha.',.alpha .'-bis(4"-hydroxyphenyl)ethyl]benzene, and 1-[.alpha.-methyl-.alpha.-(2',4'-dihydroxyphenyl)ethyl]-3-[.alpha.",.alpha .'-bis(4"-hydroxyphenyl)ethyl]benzene; and PA1 another polyhydroxy compounds such as p-bis(2,3,4-trihydroxybenzoyl)benzene, p-bis(2,4,6-trihydroxybenzoyl)benze ne, m-bis(2,3,4-trihydroxybenzoyl)benzene, m-bis(2,4,6-trihydroxybenzoyl)benzene, p-bis(2,5-dihydroxy-3-bromobenzoyl)benzene, p-bis(2,3,4-trihydroxy-5-methylbenzoyl)benzene, p-bis(2,3,4-trihydroxy-5-methoxybenzoyl)benzene, p-bis(2,3,4-trihydroxy-5-nitrobenzoyl)benzene, p-bis(2,3,4-trihydroxy-5-cyanobenzoyl)benzene, 1,3,5-tris(2,5-dihydroxybenzoyl)benzene, 1,3,5-tris(2,3,4-trihydroxybenzoyl)benzene, 1,2,3-tris(2,3,4-trihydroxybenzoyl)benzene, 1,2,4-tris(2,3,4-trihydroxybenzoyl)benzene, 1,2,4,5-tetrakis(2,3,4-trihydroxybenzoyl)benzene, .alpha.,.alpha.'-bis(2,3,4-trihydroxybenzoyl)-p-xylene, .alpha.,.alpha.',.alpha.'-tris(2,3,4-trihydroxybenzoyl)mesitylene, PA1 2,6-bis(2'-hydroxy-3',5'-dimethylbenzyl)-p-cresol, 2,6-bis(2'-hydroxy-5'-methylbenzyl)-p-cresol, 2,6-bis(2'-hydroxy-3',5'-di-t-butylbenzyl)-p-cresol, 2,6-bis(2'-hydroxy-5'-ethylbenzyl)-p-cresol, 2,6-bis(2',4'-dihydroxybenzyl)-p-cresol, 2,6-bis(2'-hydroxy-3'-t-butyl-5'-methylbenzyl)-p-cresol, 2,6-bis(2',3',4'-trihydroxy-5'-acetylbenzyl)-p-cresol, 2,6-bis(2',4',6'-trihydroxybenzyl)-p-cresol, 2,6-bis(2',3',4'-trihydroxybenzyl)-p-cresol, 2,6-bis(2',3',4'-trihydroxybenzyl)-3,5-dimethylphenol, 4,6-bis(4'-hydroxy-3',5'-dimethylbenzyl)pyrogallol, 4,6-bis(4'-hydroxy-3',5'-dimethoxybenzyl)pyrogallol, 2,6-bis(4'-hydroxy-3',5'-dimethylbenzyl)-1,3,4-trihydroxyphenol, 4,6-bis(2',4',6'-trihydroxybenzyl)-2,4-dimethylphenol, and 4,6-bis(2',3',4'-trihydroxybenzyl)-2,5-dimethylphenol.
Use of compound [VI] gives a tetraester in high yield, use of compound [VII] gives a hexaester in high yield, and use of compound [VIII] or [IX] gives a diester in high yield.
Further examples thereof include the following compounds [X] to [XII] and compounds [XIII] to [XVI], which are described in JP-A-5-323597 and JP-A-6-167805. ##STR4##
Use of compounds [X] to [XVI] gives diesters in high yield.
Examples of the selectively esterifiable compound further include the following compounds. ##STR5##
Use of compounds [XVII], [XX], [XXI], and [XXII] gives diesters in high yield, while use of compounds [XVIII] and [XIX] gives triesters in high yield.
On the other hand, 1,2-naphthoquinonediazide-5(and/or -4-)sulfonic ester compounds widely used as a photosensitive material for positive working photoresists are being synthesized by the esterification reaction of a polyhydroxy compound with 1,2-naphthoquinonediazide-5-(and/or -4-)sulfonyl chloride in the presence of a base catalyst (e.g., an inorganic base such as sodium hydroxide, sodium carbonate, or sodium hydrogencarbonate or an organic base such as triethylamine or dimethylaminopyridine).
In general, the esterification reaction of a polyhydroxy compound with 1,2-naphthoquinonediazide-5-(and/or -4-)sulfonyl chloride yields a photosensitive material in the form of a mixture of various esters which differ in the number and positions of ester bonds. However, it was found that when a base catalyst such as those enumerated above, e.g., triethylamine, is used to conduct the esterification of polyhydroxy compounds [III] to [XXII], esters of photosensitive materials containing specific unreacted hydroxyl group(s) can be synthesized in a certain selective manner.
Furthermore, as a result of intensive studies made by the present inventors, it has also been found that an esterification reaction having a higher selectivity for the desired ester of the photosensitive material having hydroxyl group(s) in the molecule enables to attain a more satisfactory resist performances. More particularly, it has been found that when positive working photoresists containing the photosensitive materials obtained by the esterification reactions described above are evaluated for performances, the resists containing the photosensitive materials obtained by the esterifications having a higher selectivity for the desired ester are excellent in the film thickness dependence of resolving power, development residue, etc.